Conventionally, in a fixing device, the toner image (unfixed image) formed on the recording material (hereafter referred to as a sheet or paper) is subjected to a fixing process by heat and pressure.
In the market, image formation on an envelope as the recording material has been desired. For example, in a device described in Japanese Laid-Open Patent Application 2014-025965, in order to prevent generation of a crease on the envelope, a constitution in which a pressure is made small compared with when an ordinary recording material, such as plain paper, is used, is employed.
Here, the envelope is a recording material that has a bag shape such that two sheets are partly pasted, or the like, and in which a thick portion and a thin portion exist in mixture. For example, an envelope that is called Choukei 3 you includes one that is laminated at a central portion of a short side and is pasted (central bonding), and one that is laminated at an end portion of the short side and is pasted (end bonding). The laminated portion requires a margin (tab) for pasting, and, therefore, is configured so that three sheets are partly superposed.
At such a laminated portion, compared with a thin portion where the sheets are not laminated, much heat of the fixing device is taken. As a result, there is a liability that a heat quantity is insufficient and unfixed toner cannot be sufficiently fixed, so that there is room for improvement.